In electrophotography, the copying speed tends to increase further in recent years to meet the ever increasing quantity of information to be dealt with. On the other hand, it is desired in electrophotography that copied marks have the same quality from the first copy to the several ten thousandth copy.
In order to obtain vivid marks even in high-speed copying, it has conventionally attempted to improve toner resins by placing importance on heat-melting characteristics such as low-temperature fixing property and offsetting resistance as well as the electrification stability of toners.
With a view toward attaining such a goal, the present inventors have already developed a new technique in which a styrene resin having a lower molecular weight (Mn: 1,000-5,000) is used as a toner resin (PCT/JP 87/00819).
However, polyvinyl chloride (PVC) resistance has recently come to the surface as a new problem. When a sheet of paper bearing a price or the like copied thereon by way of example is left over under a load on a PVC sheet for one week, some of the marks copied is caused to transfer to the PVC sheet and for example, one zero (0) disappears from the copied paper sheet. This will certainly causes a serious problem because the price has been reduced by one figure. Further, the PVC sheet has been smeared by the sticky tone deposited thereon, thereby causing another problem.
As a result of a follow-up test, it was found that the above-mentioned technique developed by the present inventors is not fully satisfactory in PVC resistance although it is extremely good in low-temperature fixing property, offsetting resistance, mark vividness and other properties. As possible reasons for this insufficiency, it may be mentioned that since the styrene-base resin has strong absorbing capacity for a plasticizer such as dioctyl phthalate in the PVC sheet, the plasticizer is caused to migrate into the toner and the toner is plasticized as a result.